A monitoring system in which terminal devices can communicate with a center device has been traditionally provided (see Japanese Patent Application Publication No. 2006-54832). In order to monitor or control the operations of apparatuses (monitored devices), each terminal device is provided with an apparatus(es), and the system transmits and receives monitoring or control information of the devices between the center device and the terminal devices.
In this sort of monitoring systems, there is a generally employed configuration in which a center device performs polling with respect to each of terminal devices and the center device obtain monitoring or control information of apparatuses from the terminal devices.
On the other hand, there is a configuration in which, in case a router is provided at the side of a terminal device(s) between a center device and the terminal apparatus(es) and the communication from the center device to the terminal device(s) is prohibited, request information can be transmitted from the center device to the terminal device(s) only by responding a request (question) from the terminal device(s) to the center device.
Timing for receiving request information per terminal device is defined as a constant (transmission time) interval in general in configurations such as, as presented above, the configuration in which the center device performs polling with respect to the terminal devices and thereby request information is transmitted from the center device to the terminal devices, or the configuration in which request information is provided from the center device to the terminal device(s) by responding to a request from the terminal device(s) to the center device. That is, timing, by which monitoring information is transmitted from each terminal device to the center device and control information is transmitted from the center device to each terminal device, is a constant (transmission time) interval.
Therefore, even if request information to be transmitted to a specified terminal device is generated in the center device, it is necessary to wait up to the timing at which another request information can be transmitted to the terminal device, and thereby a delay time occurs from the generation of the request information to the actual transmission of the request information.
In order to reduce the delay time when request information is transmitted from the center device to the terminal device, it is conceivable that the aforementioned transmission interval should be shortened. However, if the number of the terminal devices communicating with the center device is increased, each allotted time of the terminal devices becomes short. Accordingly, speeding up the communication speed is required, and the processing load of the center device is increased.